Various instruments utilize conduits, such as tubes for transportation of process fluids and sample compounds and/or for separation of sample compounds, and optical fibers to transport light. For example, chemical-analysis instruments that utilize liquid chromatography (LC), capillary electrophoresis (CE) or capillary electro-chromatography (CEC) perform separation of sample compounds as the sample passes through a column, or concentrate a sample in a trap column before delivery of the concentrated sample to a separation column. Such instruments include plumbing, such as tubes and connectors, that transport a variety of materials, such as solvents and sample compounds.
In addition to tubing, liquid-chromatography apparatus typically include, for example, reservoirs, pumps, filters, check valves, sample-injection valves, and sample-compound detectors. Typically, solvents are stored in reservoirs and delivered as required via reciprocating-cylinder based pumps. Sample materials are often injected via syringe-type pumps.
In some cases, separation columns include one or more electrodes to permit application of a voltage to a sample-containing fluid passing through and/or exiting from the conduit. CEC, for example, utilizes an electro-osmotic flow (EOF) to propel a mobile phase through a chromatographic column. In contrast, high-performance liquid chromatography (HPLC) relies on pressure to propel a fluid through a column.
Suitable tubing withstands pressures encountered during fabrication and use, is reliable through repeated use, and has physical and chemical compatibility with process and sample compounds. Generally, a tubing material should not corrode or leach, and sample compounds should not adhere to the tube (unless required for a separation process.)
For HPLC and higher-pressure applications, tubing is typically made from stainless steel or fused silica to provide suitable strength and cleanliness. Such tubing is typically joined to other components via stainless steel connectors.
Stainless steel, however, has disadvantages in some applications due to its biocompatibility limits in comparison to some other materials; some organic molecules tend to adhere to the inner walls of steel tubing, and components of a steel alloy at times leach into fluid passing through the tubing. Organic molecules generally are less likely to stick to fused silica or suitable polymeric materials than to steel. Fused silica tubing, however, is vulnerable to fracturing while polymeric materials generally have relatively poor strength.
Typically, tubing must also be compatible with connectors that provide fluidic connections to other components of an instrument. Problems associated with the design and use of connector fittings are particularly difficult for high-pressure fabrication and operation. For example, pressures in the range of 1,000-5,000 pounds per square inch (psi) or higher are often utilized in liquid chromatography, and must be accommodated without undesirable amounts of leakage. Tubing connections should generally minimize dead volume, a problem that grows worse as dimensions are reduced.